This invention relates to a device for detecting an amount of remainder of a bobbin thread wound up around a bobbin of a sewing machine particularly for detecting a state in which the remainder of the bobbin thread reduces to an amount less than a predetermined amount.
In prior art, there is known a bobbin thread remainder detecting device including a photo-detector disclosed in Japanese Utility Model Laid-open (KOKAI) Publication No. 61-180685, in which when a rotation of a rotating hook is stopped, a light is irradiated to the outer periphery of a bobbin shaft through a hole form on a bobbin case and a reflected light of the irradiation is detected by the photo-detector. The bobbin thread remainder detecting device discriminates whether or not the remainder of the thread is less in amount than the predetermined amount in response to a detection value generated by the photo-detector, and determines whether or not the bobbin should be exchanged when the discrimination is affirmative.
In U.S. Pat. Application Ser. No. 07/692,936, now U.S. Pat. No. 5,103,750 assigned to the assignee of the present application, a bobbin thread remainder detecting device which includes a light detector is disclosed. In the bobbin thread remainder detecting device, the light detector comprises a pair of light receiving elements, for receiving the reflected light from a bobbin shaft, disposed at different positions for the reason that if the reflected light is detected by only one light receiving element, the remainder of the bobbin thread may not be exactly detected because of reduction of a sensitivity due to adhesion of dust or oil to a light detecting portion of the light receiving element. In the arrangement of the paired light receiving elements, the bobbin thread remainder is detected in accordance with the ratio of the received light amounts of both the receiving elements, so that the bobbin thread remainder is detected without an effect of reduction of the sensitivity.
However, it is considered that, according to the bobbin thread remainder detecting device utilizing the light detector of the structure described above, bobbin threads to be wound up around the bobbin generally have various colors sizes, thicknesses, glosses and the like, and the reflected conditions of the light are significantly and minutely different in accordance with these features of the bobbin threads to be used. This may result in that the amount of the remainder of the bobbin thread calculated by the above mentioned method does not correspond to the really exact amount thereof, and in fact, in the experiments, there obtained the different results in the discrimination of the calculated results, made in accordance with the above mentioned method, as to whether or not the calculated bobbin thread remainder amount is less than the predetermined amount in spite of the fact that the same amounts of the bobbin threads in fact remained in different cases.
Furthermore, in the bobbin thread remainder detecting device described above, since the remainder is detected by detecting the mass of the bobbin thread wound up around the bobbin, the lengths of the actual bobbin thread remainders are different in dependency on the size or thickness of the bobbin thread when it is the discriminated that the bobbin thread remainder is less than the predetermined amount.